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Potential Deadlocks During Concurrent Write

Whenever transactions involve updates of more than one table, there is always the possibility of concurrently running transactions becoming deadlocked when they both try to write to the same set of tables. A transaction releases all of its locks at once when it either commits or rolls back; it does not relinquish locks one at a time. For example, suppose that transactions T1 and T2 start at roughly the same time.

  • If T1 starts writing to table A and T2 starts writing to table B, both transactions can proceed without conflict; however, if T1 finishes writing to table A and needs to start writing to the same rows as T2 does in table B, it will not be able to proceed because T2 still holds the lock on B. Conversely, if T2 finishes writing to table B and needs to start writing to the same rows as T1 does in table A, it will not be able to proceed either because T1 still holds the lock on A. In this case, a deadlock occurs. If T1 is committed and releases the lock within the lock timeout duration, subsequent update will proceed. If a lock times out, an error will be reported and the corresponding transaction will exit.
  • If T1 updates rows 1 to 5 and T2 updates rows 6 to 10 in the same table, the two transactions do not conflict. However, if T1 finishes the update and proceeds to update rows 6 to 10, and T2 proceeds to update rows 1 to 5, neither of them can continue. If either of the transactions is committed and releases the lock within the lock timeout duration, subsequent update will proceed. If a lock times out, an error will be reported and the corresponding transaction will exit.